1. Field of the Invention
This invention relates to a switching amplifier including a signal converter controlling plural series-connected switching stages, in which the total possible voltage range of the analog input signal is subdivided into a plurality of identical voltage steps and each voltage step is associated with a switching stage and in which the output voltages of switching stages are series-connected and summed in the exact number as correspond to the number of voltage steps within the respective amplitude of the analog input signal.
2. Description of the Prior Art
Such a switching amplifier as above noted is known, for example, from German Offenlegungsschrift No. 30 44 956. As an output signal, a time-related voltage is generated in the form of a staircase which, after smoothing in a low-pass filter, approximates the appropriately amplified analog input signal.
The amplifier described has already been successful particularly as a modulation amplifier in power transmitters and contributed to a significant improvement in the transmitter efficiency with respect to the electrical energy used.
An essential feature of the known amplifier is the fixed association between the voltage steps of the input signal range and the corresponding switching stages on the output side. Every time the amplitude of the input signal increases and reaches a certain voltage step, a certain switching stage associated with this voltage step is connected into the series circuit, that is to say its output voltage is also included into the summation occurring at the output. Every time the amplitude of the input signal, on the other hand, decreases and drops below this certain voltage step, the same switching stage is switched out of the series circuit so that its output voltage no longer contributes to the summation process. If, in contrast, the amplitude is always below or above, respectively, this voltage step, the associated switching stage remains constantly switched out or in.
Since in normal amplifying operations that occur, for example, in a broadcast transmitter, very small but also very large signal amplitudes occur relatively less frequently in the analog input signal than medium amplitudes because of a statistical distribution, switching processes occur relatively less frequently in the switching stages associated with those amplitude ranges. In other words, the switching stages responsible for small amplitudes are switched in for the predominant period of time and the switching stages responsible for high amplitudes are correspondingly switched out for the predominant period of time. In agreement with the unequal amplitude distribution, unequal operating conditions also exist therefore in the known amplifier for the different switching stages because of the rigid association of each switching stage with a corresponding amplitude.
To this unequal distribution of the switching processes, another very grave inequality is added in the reloading of the switching stages resulting from the principle of series connection. More particularly, the more switching stages are switched in series when the load at the output remains the same, the higher the voltage sum of the series-connected output voltages and thus also the current flowing through each switching stage. If thus in an already existing series circuit, one switching stage of many switching stages is switched in or out, a much higher current must be switched within this stage than in those stages which are located at the beginning of a series circuit. Accordingly, the current load on the switching elements in the switching stages is the greater the higher the input signal amplitude of the voltage steps associated therewith so that, simplified, in the fixed association according to the state of the art the upper switching stages are always subject to a high switching load whereas the lower switching stages carry a low switching load.
Finally, in the known amplifier very short switching-in or -out periods occur under certain circumstances for the individual switching stages, which periods require an unnecessarily high switching frequency for the switching elements used in the switching stages so that, for example, thyristors can be used in only a limited way because of the delay processes caused internally and by the external circuit elements. If, for example, a higher-frequency input signal, the amplitude of which varies only between two adjacent voltage steps, is amplified, the associated switching stages are switched in and out with the same higher frequency whereas other switching stages remain consistently switched in or out.